The present invention relates to oil-free dry screw compressors and methods of controlling oil-free dry screw compressor systems.
Compressor systems employing induction drives are known. Limitations on the frequency of starting and stopping induction motors have posed limitations on the ability of compressor systems using such motors to conserve energy. As a result, many induction drive compressor systems employ continuously running drives and motors and control pressure in the system by frequently releasing built up pressure through a blowdown valve. Such a design does not provide a system that can control pressure over an entire 0% to 100% demand range.
Although variable speed pumps and drives are known, for example, from U.S. Pat. Nos. 5,522,707 and 3,216,648, which are both incorporated herein in their entireties by reference, a need exists for a variable speed compressor of an oil-free type that includes a drive and motor system capable of being shut off or stopped and restarted an unlimited number of times during any given time period. A need also exists for a method of controlling pressure in a compressor system whereby a constant pressure can be maintained over an entire 0% to 100% demand range.
The present invention provides an oil-free dry screw compressor having a variable speed drive. More particularly, the present invention provides an oil-free dry screw compressor system preferably having two or more compression stages. Preferably, the system includes a rectifier/inverter drive that can rectify an alternating current to a direct current and invert a direct current to an alternating current, and an electric motor with controls to start and stop the motor an unlimited number of times over a given time period. Preferably, the compressor system includes two or more airends or stages that are free of conventional inlet valves. The two or more airends or stages are preferably driven by a single variable speed drive and motor through a gear system that provides simultaneous compression in both airends or stages.
The present invention also relates to a method for controlling the pressure of a compressed fluid produced by an oil-free dry screw compressor. The method includes compressing a fluid with an oil-free dry screw compressor driven by a variable speed drive, flowing compressed fluid generated by the oil-free dry screw compressor through a compressed fluid conduit, sensing the pressure of compressed fluid in the compressed fluid conduit, and adjusting the speed of the variable speed drive in response to the pressure sensed in the compressed fluid conduit.
It is to be understood that both the foregoing general description and the following detailed description are exemplary and explanatory only and are only intended to provide a further explanation of the present invention, as claimed. The accompanying drawings, which are incorporated in and constitute a part of this application, illustrate several exemplary embodiments of the present invention and together with description, serve to explain the principles of the present invention.